remove superfluous stream

There is a stream allocation at line 170 that seems needless:
https://github.com/dominictarr/JSONStream/blob/master/index.js#L170
since at line 173 there is another stream assignment that overwrites the previous value:
https://github.com/dominictarr/JSONStream/blob/master/index.js#L173

It seems like this was a bugfix for an odd ordering bug because when I removed that allocation without also updating the inner through functions to use `this` instead of `stream`, it didn't work. Now everything is fine.